An internal combustion engine has an engine valve for opening and closing intake and discharge ports. The engine valve comprises a circular head that closes the intake port or the discharge port, and a rod stem, which is connected to the head and held by a cylinder head of the internal combustion engine. The stem part of the engine valve slidingly moves quickly with respect to the cylinder head in order to open and close the intake and discharge ports. Thus, a surface-modifying treatment, such as a nitriding treatment, is applied to the engine valve in order to improve abrasion resistance.
Known nitriding treatments for engine valves include a gas nitriding method in which the engine valve is heated in an ammonia gas atmosphere, a salt bath nitriding (tuftriding) method in which the engine valve is salt-bathed with a mixed salt containing sodium cyanate or potassium cyanate, and a plasma nitriding method (ion nitriding) in which the engine valve is continuously irradiated with a plasma.
For example, when a plasma nitriding treatment is applied to an engine valve formed from an iron group series alloy, the engine valve first is set on a grid portion of a specimen stand, which serves as a surface-modifying jig. Incidentally, the engine valve has a spontaneously-generated passivation film (oxide film) on the surface thereof, wherein the passivation film impedes progress of the nitriding treatment. For this reason, the passivation film is removed by hydrogen-sputtering prior to the nitriding treatment. Then, a hardened layer is formed on the surface of the engine valve by applying a continuous plasma nitriding treatment to the engine valve (e.g., see Patent Document 1).
Patent Document 1: Japanese Patent No. 3553549